1) cAMP and cGMP, are critical intracellular messengers, under the control of exogenous agents. In NlE-ll5 cells, muscarinic agonists affect both cAMP and cGMP, causing suppression and elevation, respectively. Muscarinic suppression of cAMP levels was not blocked by cAMP phosphodiesterase inhibitors, and thus results from inhibition of adenylate cyclase and not activation of phosphodiesterase. Inhibitory effects of muscarinic agents on cAMP are mediated by a guanyl nucleotide-binding protein, Gi; the mechanism by which they influence cGMP generation is unclear. Pertussis toxin, by catalyzing the ADP-ribosylation of Gi, reduced the muscarinic suppression of basal and prostaglandin-stimulated cAMP. The toxin did not affect muscarinic agonist-dependent cGMP levels, consistent with the hypothesis that Gi is not involved in this pathway. 2) NAD:arginine ADP-ribosyltransferases, previously identified in animal tissues, catalyze reactions similar to bacterial toxins, such as choleragen, which cause activation of adenylate cyclase. Kinetics of the reactions catalyzed by choleragen and an erythrocyte ADP-ribosyltransferase were examined with agmatine, an arginine analogue, as the ADP-ribose acceptor. The data were most consistent with a random, rapid equilibrium, sequential mechanism; binding of either NAD or agmatine had a negative effect on the subsequent binding of the other substrate. 3) NAD:arginine ADP-ribosyltransferases catalyze the ADP-ribosylation of arginine and proteins. An enzymatic activity was identified in erythrocytes that cleaved ADP-ribosylarginine to ADP-ribose and arginine. The reaction was stimulated by divalent cations (e.g., Mg2+) and thiols. A product of the reaction, arginine, was, in the presence of NAD, ADP-ribosylated by choleragen and erythrocyte transferase. Since deradation of ADP-ribosylarginine generates an arginine moiety that is a substrate for the transferases, it appears that ADP-ribosylation may be a reversible covalent modification.